A Suggestion on How to Get Substorm Research Moving Again

C. T. Russell

Department of Earth and Space Sciences and Institute of Geophysics
and Planetary Physics University of California, Los Angeles, CA 90024

Originally published in:Strategies for the Tail and Substorm Campaign, edited by W.J.
Hughes, 24-31, Boston University Center for Space Physics, 1994.

1. Introduction

The realization that the interplanetary magnetic field (IMF) joins with
the dayside terrestrial magnetic field when the IMF is southward is over
3 decades old [Dungey, 1961]. The complementary idea of connection
between the IMF and magnetotail magnetic field when the IMF is northward
[Dungey, 1963]. In these so-called reconnecting magnetospheres,
the rate at which magnetic flux becomes linked governs the rate of circulation
of the plasma inside the magnetosphere, i.e. the magnetospheric electric
field. The ratio of the potential drop across the magnetosphere due to
reconnection to the potential drop across an equivalent distance in the
solar wind flow might be considered to be the efficiency of reconnection.
The flow in this model is steady state. There is no net transfer of magnetic
flux or plasma.

It was not long before time dependence was added to the Dungey model
to generate a model for magnetospheric substorms. This model has been called
the near-earth neutral point model of substorms [McPherron et al.,
1973; Russell and McPherron, 1973]. This model is illustrated in
Figure 1.

Fig. 1 Schematic representation of the near Earth neutral point model of
a substorm. At the beginning of the growth phase the IMF turns southward
and the merging rate at the dayside magnetopause, M, increases. The reconnection
rate in the tail, R, does not increase until some time later. Thus, the
amount of magnetic flux in the tail lobe varies with time. Rate C represents
the convection of plasma from the tail to the dayside magnetosphere.

Initially the IMF is northward and the magnetosphere is in a quiescent
equilibrium state. Then the IMF turns southward, reconnects with the dayside
closed magnetic field at a rate, M, and transfers magnetic flux to the
magnetotail. This decreases the magnetic flux on the dayside, and increases
the magnetic flux in the lobes of the magnetotail, .
If the reconnection rate in the tail, R, immediately rose to meet
that at the nose, the electric field in the magnetosphere would rise and
fall, but the magnetic flux in the various regions of the magnetosphere
would remain fixed. However, the reconnection rate in the tail is not immediately
responsive to the nose reconnection rate and magnetic flux in the tail
builds up until such (explosive?) reconnection begins. At that point the
rate of convection of magnetic flux from the night plasma sheet to the
dayside, C, increases depleting the initial buildup of closed flux in the
plasma sheet, FPS. If reconnection begins in the tail on initially
closed magnetic field lines, a plasmoid or magnetic bubble is formed that
is ejected tailward [Russell, 1974].

If there has been any progress in this area in the intervening two decades,
the progress is that reconnection has now become generally accepted as
an important magnetospheric process. This acceptance became general after
it was quantitatively verified at the dayside magnetopause [Paschmann
et al., 1979]. Thus, essentially all popular substorm models invoke
a cycle of reconnection, and the debate over the difference between substorm
models has evolved to a discussion of subtle differences in the timing
of the sequence of events occurring during the substorm [Fairfield
1991; Kennel, 1992]. Nevertheless despite general acceptance of
reconnection, there is a real remaining substorm problem: what process
determines the moment of auroral breakup that by definition [Rostoker
et al., 1980] defines the onset of the expansion phase of a substorm.
This process could be a global instability, a local instability with global
consequences or an external trigger. It is not that ideas are lacking.
In fact, several authors have demonstrated that the number of possible
models exceeds the number of researchers [e.g., Baker et al., 1982;
Baker and McPherron, 1990]. The problem, as I see it, is that development
of the tools for solving the problem has been ignored. It is the purpose
of this note to suggest an approach to resolving this impasse.

2. Not the Road to Progress

Before discussing the positive steps that need to be followed to bring
the substorm onset problem to closure, we will mention a few paths that
seem to be blind alleys. The first blind alley exists because substorms
have a bad name. This bad name has nothing to do with the quality of research
done in this field. Rather, the name is bad because substorms are not small
geomagnetic storms or the basic building blocks of geomagnetic storms.
The intensity of geomagnetic storms seems to be quite independent of the
strength of "substorm" activity in the auroral oval [Russell
et al., 1974; Feldstein 1992]. The conditions in the solar wind
that lead to storms are prolonged strong southward IMF and high solar wind
velocities, while the conditions that lead to substorms are short (~1 hour)
southward IMF periods with small (~5 nt) southward fields. The former conditions
lead to a buildup of the ring current, but the latter usually do not. Substorms
are usually best studied when they are isolated from other activity. Storms
can only be studied when activity is high for a very extended period (~days).

Progress has been made in the study of substorms through Coordinated
Data Analysis Workshops (CDAWs). While this progress is very welcome, the
progress has come at a large price for both the researchers involved and
the community at large, as the workshops have become very large and unwieldy.
Many researchers have the tools to analyze these data, and they do not
have to be studied in large community efforts. These data need to be released
much more rapidly to the community at large for individual analysis. Large
communal efforts frequently lead to lowest common denominator science and
slow the publication of any results.

A regular series of meetings maintains the appearance of progress, but
it does not always signal the existence of real progress. It is commendable
that the substorm community wishes to converse as often as they do about
work on the substorm problem and has established a biennial International
Substorm Conference on top of all the other meetings in the field that
treat substorms, but a skeptic might also view this as evidence that the
community is more interested in traveling than in working, especially since
the same ideas about substorm processes are being debated today as were
being debated twenty years ago. One cynic has even suggested a conspiracy
theory of substorms, that the principals have agreed not to solve the problem
in order to keep the field alive.

3. A Quantitative Approach

Most researchers today would say that it is clear that substorms come
in a variety of sizes. In fact, the hotly debated topic of pseudo-breakup
[Koskenin et al., 1993] may be simply a question of size. Perhaps
events below a critical size do not proceed through the full cycle of substorm
associated changes. But what is that size? The present acceptance of the
existence of varying substorm sizes was not always so. The 1978 Victoria
conference on the definition of substorms refused to consider size as an
important issue in substorms. In fact, the author distinctly remembers
one very senior scientist stating flatly that substorms did not have different
sizes. This statement is true in a technical sense because there is no
accepted measure of the size of a substorm despite over 30 years of research
on substorms. Atmospheric storms and hurricanes have a measure of size
based on the strength of their winds. Earthquakes have measures of size
depending on either the destruction caused or the energy released. Solar
flares have measures of size for each of several wavelengths but substorms
have no measure of strength. Plots of AE are generally now shown, but the
AE index is not a good measure when the substorm is small and is poleward
of the usual longitudinal AE chain of stations or if it is large and equatorward
of the AE chain. Also, the AE chain is very uneven in its longitudinal
coverage. Thus, researchers seem to have shied away from quantifying substorms
by their peak AE response. Substorm researchers do not talk about a 500
nT substorm, a 1000 nT substorm, etc.

If we are to make progress in studying the magnetosphere, to test our
understanding and our models, we need to understand quantitatively how
the current systems respond to changing solar wind conditions. These currents
include the Chapman-Ferraro current, Birkeland currents, tail currents
and substorm currents. It is clear that these currents do vary with solar
wind conditions. For example, as illustrated in Figure 2 the position
of the magnetopause moves inward when the IMF is southward, an amount proportional
to the southward component of the IMF [Petrinec and Russell, 1993a].

Fig. 2 The standoff distance of the nose of the magnetosphere as a function
of the north-south component of the IMF. The positions are normalized to
typical solar wind dynamic pressure conditions [Petrinec and Russell, 1993a]

This effect is presumably caused by the increase in the dayside Birkeland
current system [Russell et al., 1974b]. The response of the surface
magnetic field to sudden changes in the solar wind dynamic pressure is
less when the IMF is southward than when it is northward {Russell and
Ginskey, 1993]. Again this could be due to an increase in the region
1 currents but also tail current increases could contribute. The clearest
evidence that the tail currents increase when the IMF is southward is that
the strength of the field in the tail lobes increases due to increased
flaring of the tail boundary in proportion to the southward component of
the IMF [Petrinec and Russell, 1993b]. This proportionality is shown
in Figure 3. These studies begin to quantify the dependence of the magnetospheric
currents on the IMF but much more needs to be done.

Fig. 3 The square of the sine of the flaring angle of the tail at approximately
20 RE as a function of the north-south component of the IMF.
The flaring angle has been normalized to typical solar wind dynamic pressure
conditions [Petrinec and Russell, 1993b].

Two studies that have addressed the effects of substorms on the current
systems (but not the effect of IMF on the substorm) have been carried out
by Pulkkinen et al. [1993] and Chun and Russell [1991]. The
former study examined how the tail current moved and was enhanced during
a substorm. The latter study examined how region 1 or 2 currents varied
on average during a substorm. Again, these studies are only a beginning.
Much more needs to be done on both topics.

In addition to quantitative studies of the control of the currents by
the solar wind and thus control of the currents by the substorm, an important
quantitative study that is crying to be done is what are the solar wind
conditions that lead to a substorm onset. We know from Kokubun et al.
[1977] that a sudden impulse can trigger a substorm if the IMF has
been southward before the sudden pressure change reaches the magnetosphere
but in the absence of a sudden impulse what conditions are needed? Does
the IMF have to be 5 nT southward for an hour? If so, would 10 nT southward
for a half hour be just as effective? Figure 4 shows examples of the IMF
signature prior to four substorms [Caan et al., 1977]. This work
suggests that s northward turning after a southward period can lead to
a substorm onset, but no attempt was made to relate the strength of the
subsequent goemagnetic activity to the interplanetary or geomagnetic conditions.
We need to pursue such cause and effect studies.

Fig. 4 The response of near midnight ground-based magnetic records to southward
and northward turnings of the IMF. These substorms appear to be triggered
by northward turnings [Caan et al., 1977].

The work of Pulkkinen et al. [1992], Chun and Russell
[1992] and Petrinec and Russell [1993a,b] among others demonstrates
that we have the tools to undertake quantitative studies of the changes
in the magnetosphere that take place at substorm times. If we are to go
beyond this work we need to have a quantitative measure of the size of
the associated substorm effects. There are many possibilities. By default
the peak AE index is being used now, at least on occasion. It would seem
more appropriate to calculate the intergrated current flowing through the
auroral oval as another measure, and the time integrated current over the
course of the substorm. Another possible measure would be the size of the
mid-latitude positive bay. Again one could look at the maximum disturbance
during the substorm, examine the spatial extent of the disturbance and/or
look at the time integrated disturbance, the area of the bay. It is not
clear a priori which parameter would be best. Perhaps all measures might
reveal different information about the substorm. All should therefore be
examined.

4. Recommendations

How can a program like GEM help to get substorm research moving again?
First, the working groups should concentrate on determining what are the
true outstanding questions. It matters little to the progress of the field
who proposed what model when. It does matter if there is a process that
once started inevitably leads to the sequence of processes seen in a substorm.
Since there is a limited amount of funding, lists of prioritized studies
could be prepared. Individual researchers could always argue (in their
proposals) with these lists but they would have to make cogent arguments
if they wanted to pursue different directions. In particular I would expect
that such rational lists would emphasize quantitative studies and hypothesis
testing and would be a very positive force for improvement.

Numerical modeling can play a very salutary role in these studies because
by its very nature it is quantitative. Efforts that take the observed time
sequence of interplanetary conditions and predict magnetospheric response
such as the MHD modeling at NRL under Joel Fedder are proving very profitable.

Finally, we need to have an approximate measure of the size of substorms.
It is clear that substorms can have a variety of strengths. Until we learn
to distinguish strong substorms from weak ones we may be pursuing minor
processes in the magnetosphere as much as the major processes and comparing
apples and oranges. For if we do not make progress soon in substorm research,
there will be more converts who believe the conspiracy theory of substorms.

Acknowledgements

This polemic was supported by the National
Science Foundation under research grant ATM 92-13379.